Inside the Quark

It wasn’t long ago that physicists were toasting the discovery of the top quark, the last, elusive piece of their standard model of matter. But now the same team that discovered the top quark has announced experimental results that--if confirmed--may mean physicists’ quest for a comprehensive understanding of matter is far from over. There is a chance that the quark itself might be composed of still smaller particles. It’s possible that we are beginning to see new physics in this experiment, says physicist Steve Geer of Fermi National Accelerator Laboratory. From an experimentalist’s point of view, it’s fun.

From the beginning of this century, physicists have explored the structure of matter by shooting tiny bits at other bits and watching how they react when they collide. They’ve found that an atom has a nucleus, that a nucleus contains protons and neutrons, and that those particles in turn are made of quarks and gluons--particles that bind quarks together. But most physicists believe quarks to be the smallest building blocks of matter.

Geer and his colleagues at Fermilab accelerated protons and their antimatter counterparts, antiprotons, to near the speed of light and slammed them together. At that speed, the particles usually sail right through one another--protons and antiprotons are, after all, mostly empty space, so the quarks and gluons inside them just pass like naval squadrons in the night. But occasionally two quarks or a quark and a gluon meet head- on and go squirting out to one side, along with other particles created by the energy of the collision.

The standard model predicts how many such collisions should occur. But at the highest energies, Geer says, we see something like 50 percent more collisions than we would expect from standard model predictions.

What’s going on? It’s too early to tell, although there is no dearth of speculation. Some theorists have suggested that the standard model description of the force that binds quarks together may need to be rewritten; others think quarks may be hitting a never-before-detected, short-lived particle called Z¢, a particle thought to be involved in the weak force, which governs radioactive decay. Geer says it’s possible that the quark itself may consist of still smaller pieces. It would certainly be nice, he says, to get to learn physics beyond the standard model.